318 THE MUSCULATURE 



annulus fibrosus, may hold a tendon in place, as, for instance, the trochlea of the 

 tendon of the superior oblique muscle of the eye. 



Synovial bursse [bursae mucosae]. — Where there is freedom of action between 

 muscles and tendons and the surrounding parts, there intervenes a loose connective 

 tissue. In regions where the pressure is great or considerable friction would re- 

 sult were these conditions retained, there are developed special cavities with 

 smooth surfaces and containing fluid. Most of these bursse are developed from 

 the intervening connective tissue at a period in embryonic life preceding muscular 

 activity, but special bursse may later be developed as the result of unusual pressure 

 or muscular activity after birth. An instance of a bursa lying in a region of fric- 

 tion may be seen in the bur?a intervening between the tendinous posterior surface 

 of the ilio-psoas muscle and the ilio-femoral ligament. As an instance of a bursa 

 lying in a region of intermittent pressure may be cited that between the tendon of 

 Achilles and the calcaneus. 



Most synovial bursae intervene between a tendon and a bone, a tendon and a 

 ligament, or between two tendons (subtendinous bursae mucosae). Others lie be- 

 tween two muscles, a muscle and some skeletal part, or between a muscle and a 

 tendon (submuscular bursae mucosae) ; or below a fascia (subfascial bursae mu- 

 cosae). Subcutaneous bursae have been referred to in connection with the tela 

 subcutanea (see p. 313). Most bm-sae are developed near joints. The bursse may 

 so expand during active life that they come to communicate with other bursae or 

 with a neighbouring joint cavity. 



Synovial sheaths [vaginae mucosae tendinum]. — Synovial sheaths are developed 

 about tendons where the latter are confined in osteo-fibrous canals, as in the 

 fingers. The wall of the canal and the enclosed tendon, or tendons, are each 

 covered by a smooth membrane which at the extremities of the canal is reflected 

 from the wall to the tendon. Between the membrane covering the tendon and 

 that lining the canal is a synovial cavity. An interesting feature of these tendon- 

 sheaths is the presence of mesotendons, delicate bands of vascular connective 

 tissue which run in places from the osseous groove to the tendon and carry blood- 

 vessels and nerves. 



Trochleae. — Where a tendon passes at an angle about a bone, the tissue in the groove in 

 which the tendon runs frequently is composed of hyaUne cartilage instead of bone. An example 

 may be seen in the trochlear process of the calcaneus. 



Nerves.— To each muscle of the body a nerve containing motor and sensory 

 fibres is distributed. A few muscles receive two or more nerves. Sherrington has 

 estimated that in the muscle-nerves of the cat two-fifths of the fibres are sensory 

 and three-fifths motor. 



The muscles of the head and in part those of the neck are supplied by branches 

 of the cranial nerves. The intrinsic muscles of the neck, back, thorax, and abdo- 

 men are supplied by branches which arise fairly directly from the spinal nerves. 

 The muscles of the limbs are supplied by branches from nerve-trunks which arise 

 from plexuses formed by the spinal nerves in the regions near which the limbs 

 are attached. 



The main nerve-trunks lie beneath the superficial muscles. They usually run in the 

 intermuscular septa which separate the deeper groups of muscles from one another and from 

 the superficial muscles. The nerve-branches which enter a given muscle usually pass in where 

 the larger intramuscular septa approach the surface of the muscle, and then ramify through 

 the perimysium internum, the smaller branches being distributed in the finer layers of connective 

 ti.ssue which surround and separate the primary muscle fibre-bundles, to the constituent muscle- 

 fibres of which terminal l)ranches are given. Special sensory end organs are distributed chiefly 

 in the large intramiiscLdar septa, in the tendons and in the muscles near the tendons. Simple 

 sensory endings are found on the muscle fibres. 



The size of a nerve supj)lying a muscle is not proportional to the size of the latter, but 

 rather to the complexity of movements in which the muscle plays a part. 



Muscles receive their nerve supply early in development. During later development the 

 muscle may wander a considerable distance from its place of origin and carry its nerve with it. 

 The diaphragm, innervated liy cervical nerves, is a good example. 



The diHtri[)ution of the motor nerves varies according to the architecture of the muscle, but 

 in general it appears that the nerves are so distributed as to carry the main branches of distri- 

 bution most directly to tlie middle of the constituent fil)re-buiidles. This is seen most clearly in 

 muscles with comparatively short, (ihrc-bundlos, where the individual muscle-fibres run nearly 

 or quite the entire distance from tendon to tendon (fig. 338 a, c, d, e, g, h, and i). When the 

 distance is long, a marked plexiform arrangement is found (fig. 338, b and f). To each muscle 



